FIX session msg vs session tags

The “content” of FIX data is surrounded by session related data, including session Messages and session Tags. As a beginner, I was a bit confused over the difference between session Messages vs session Tags

A) There are very few (5 to 10) session message types, such as heart beat, logon/logout, request for resend…
** All the types — session msg subtypes or app msg subtypes — show up under Tag 35.

B) In ANY (session/app) Message, there are session-related Tags such as (34)seqNum, (10)checksum, (9)bodyLength, (52) timeStamp, encryption. I’d say even the all-important (35)MsgType tag qualifies as session tag.

In fact, application data are “surrounded” by session data. App data are the stars of the show, surrounded by a huge “supporting crew” —
– A session msg’s fields are 100% session-related;
– An app msg is not free of session information. It is partly a session msg — due to those session tags.
– If you snoop the conversation, most messages are session messages — heartbeats
– every business data exchange starts with and ends with session messages — logon and logout.

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